The structure and function of a low molecular weight calcium binding protein which has been purified from platelet homogenates will be determined. Results will be compared to known calcium-binding proteins from other sources, especially muscle troponin-C and cAMP phosphodiesterase calcium-dependent regulator. Structural studies include amino acid composistion and calcium-binding capacity under a variety of conditions and in the presence of agents which affect platelet function. Tertiary structure changes in the presence of calcium will be determined by spectral studies. Functional studies include similarity to troponin-C (calcium-sensitizing activity), effects on platelet phosphodiesterase, adenyl cyclase, and actomyosin activities, and sensitivity to platelet kinases. The role of the protein in intracellular calcium fluxes during stimulation-secretion coupling will be examined. Implications for the hemostatic mechanism will be considered by using drugs known to alter platelet function, and observing their effects on the protein's calcium-binding capacity and subcellular localization.